We are combining the powerful tools of recombinant DNA technology and electron microscopic visualization in order to probe some mechanisms of eukaryotic gene regulation. At present, we have evidence for the presence of a variety of regulatory DNA sequences in the "non-transcribed spacer region of Xenopus laevis ribosomal DNA (rDNA). These include a major promoter at the 5 feet initiation end of the coding region for ribosomal RNA, duplicated promoter-like sequences located several kilobases upstream in the spacer, a terminator at the 3 feet end of the gene and a failsafe terminator about 100 base pairs upstream from the major promoter. In addition, we have evidence for different amounts of the spacer being required for maximal transcription when a cloned ribosomal gene is injected into oocytes vs. embryos. Thus, in this study, we will ask the following kinds of questions: (1) What is the function of the duplicated promoter-like sequences that are present in variable numbers in the non-transcribed spacer? Does their presence influence the activity of the major promoter? In special cases, these promoters are utilized in oocytes? Are they ever used in somatic cells or embryos? (2) If two frog rDNA promoters are present on the same recombinant plasmid circle, does the turning ON of one of them affect the activity of the other? (3) We have evidence for transcriptional domains on such plasmids which are associated with changes in chromatin morphology. What sequences are responsible for the transcriptional domain and the initiation of the change in morphology? (4) What sequences regulate the maximum RNA polymerase density seen on ribosomal RNA genes?